Ultra short optical pulses can be used in a number of applications including optical information processing and data communication, optical probing with high temporal resolution, laser surgery, and material processing. In particular, recent advances in optical data communication with data rates up to 2.5 Gbit/s or higher demand compact, ultra fast light sources with low maintenance, high reliability, and low cost.
Fiber lasers have been developed as a new generation of compact, inexpensive and robust light sources. In essence, a fiber laser is an optically-pumped resonator with a section of doped-fiber as the gain medium. As the gain exceeds the total optical loss in the resonator, a laser oscillation can be generated. Many different dopants can be used to achieve laser oscillations at different wavelengths. Atomic transitions in rare-earth ions can be used to produce lasers from visible wavelengths to far infrared wavelengths (e.g., 0.45 μm-3.5 μm). Erbium-doped fiber lasers for producing optical pulses at 1.55 μm are particularly useful for optical fiber communication since the optical loss in commonly used silica fibers is minimum at about 1.55 μm.
Mode-locked fiber lasers can use various cavity configurations such as linear, ring, and figure-eight geometries. See, for example, U.S. Pat. No. 5,008,887 issued to Kafka et al. on Apr. 16, 1991 and U.S. Pat. No. 5,513,194 issued to Tamura et al. on Apr. 30, 1996. However constructed, a mode-locked fiber laser is configured to have multiple longitudinal modes that simultaneously oscillate. A mode-locking mechanism is implemented in the resonator to synchronize the phases of different modes in such a way that the phase difference between any two adjacent modes is a constant. These phase-locked modes constructively add to one another to produce a short pulse.
Two common mode-locking schemes are “active” mode locking and “passive” mode locking. Active mode locking modulates either the amplitude or the phase of the intra-cavity optical field at a frequency equal to one or a multiplicity of the mode spacing. Active mode locking can be implemented by using intra-cavity electro-optic and acousto-optic modulators.
Alternatively, passive mode locking uses at least one nonlinear optical element inside the resonator to produce an intensity-dependent response to an optical pulse so that the pulse width of the optical pulse exiting the nonlinear element is reduced. Compared to the active mode locking, passive mode locking can be used advantageously to produce ultra short light sources. Commonly-used passive mode locking techniques include saturable absorbers, figure-eight lasers and intensity-dependent nonlinear polarization rotation.
Mode-locked fiber lasers typically require a balance of “normal” (i.e., negative) and “anomalous” (i.e., positive) dispersion fibers to achieve ultra-short pulses. For lasers at short wavelengths, such as Yb fiber lasers, this requirement poses a problem. Anomalous dispersion at short wavelengths requires specialty fibers, such as bandgap fibers or photonic crystal fibers. Both of these are difficult to manufacture, problematic in splicing to conventional fiber, and further require higher order mode (e.g., LP02) propagation and appropriate mode converters at either end of the anomalous dispersion fiber.
Further, environmental stability for these sources requires that polarization maintaining fiber be used in most applications. However, the initiation of mode locking can be difficult in such laser cavities.
A need remains, therefore, for a mode-locked figure-eight fiber laser that does not require the use of anomalous dispersion material to provide the dispersion management necessary to produce mode-locked output pulses, as well as a capability of initiating mode locking in such a laser.